YOU SAID:
I don't understand why he's giving this lecture.
INTO JAPANESE
彼がなぜこの講義をしているのか理解できません。
BACK INTO ENGLISH
I can not understand why he is giving this lecture.
INTO JAPANESE
なぜ彼がこの講義をしているのか理解できません。
BACK INTO ENGLISH
I can not understand why he is giving this lecture.
Come on, you can do better than that.